This invention relates in general to radio receiver monitoring and testing circuitry, and more particularly to radio receiver monitoring and testing circuitry which determines if a radio frequency transmitted to the radio receiver is within frequency tolerances and modulation limits by monitoring and testing a detected intermediate frequency (IF) signal from the demodulator portion of a radio receiver.
In the past, the Federal Communications Commission (FCC) has required periodic testing of radio transmitting equipment for determining whether the radio freqency signal of the transmitting equipment was within the prescribed frequency tolerances and modulation limits. The frequency tolerances vary depending upon the frequency spectrum being used. Generally, the percentage of permissible error in the transmitting frequency is very small and is further dependent upon the frequency being used. For example, 0.01% error may be acceptable at 25 MHz, but this gradually decreases to 0.0005% at 150 MHz and further to 0.00015% at 810 MHz. Crystal controlled oscillators can initially satisfy such tight frequency tolerances. However, aging and other phenomena can cause the crystal frequency to drift with the passage of time, temperature changes and the like.
As is known to those skilled in the art, the radio frequencies are divided into different bands and numerous transmitting frequencies are assigned to various operators of transmitting equipment. Exacting frequency tolerances and modulation limits are required so that the transmitted signal does not cause appreciable noise or interference with the signal transmitted from an adjacent operating frequency. That is, small deviations in the transmitted signal from the center frequency will cause noise in the signal received from an adjacent center frequency, but as the deviations become greater, portions of the transmitted signal will begin to interfere with the signal at the adjacent center frequency.
Annual inspection of transmitting equipment for frequency tolerances and modulation limits is an expensive procedure. The testing equipment must have greater accuracy than the transmitting equipment to determine what degree of error is present and such sophisticated test equipment usually requires operation by trained and licensed personnel. Furthermore, testing requirements specified by the FCC generally require that the transmitting equipment be disconnected and brought to the testing location, and after the test, reinstallation is required.
Due to the ever increasing use of two way communication equipment, the burden for maintaining the accuracy of the frequency and modulation of the transmitter is now placed upon the owner. However, the FCC continues to seek out operators of offending equipment. Offenders are subject to fines, imprisonment, and loss of licenses. In addition, once an FCC citation issues, considerable additional costs are incurred in preparing replies and responses to the government.
Many businesses utilize a plurality of two way radio transmitters and receivers, often called transceivers, for maximum efficiency in utilization of manpower and equipment. In such instances, the cost of testing and maintaining the radio equipment within the prescribed specifications is multiplied by the number of transmitters in use. Similarly, the probability that at least one of the transmitting units is out of specification increases as the number of units in use increases.